Ion implantation is commonly used for doping a semiconductor material at precisely controlled depths and dopant concentrations. An ion implanter generally includes an ion source to generate an ion beam, ion beam transport optics for accelerating the ion beam, and a process chamber where the ion implantation on a semiconductor wafer occurs. The ions are mostly positively charged.
During ion implantation, the charged ion beam produces a build-up of charges on the surface of the semiconductor wafers in the process chamber. The surfaces of semiconductor wafers are generally insulating or semi-conductive. Such a charge build-up interferes with automatic wafer handling, and the implantation process itself; resulting in reduction in within-wafer uniformity (WiWU) and ultimately reduction in wafer yield.
Meanwhile, the size of semiconductor wafers has gradually increased to improve throughput and reduce cost per die. For example, in the transition from 300 mm to 450 mm wafer size, the wafer area increases by 125%. The within wafer uniformity (WiWU) becomes more difficult to maintain in the more-than-double-sized wafer.
In an ion implantation system, a plasma flood gun or system is used in front of a wafer. The plasma flood gun generates and releases electrons. These electrons neutralize the positive charged ion beam before the ion beam strikes the wafer's surface.